In order to increase the alternating current impedance of the stabiliser between the first and second terminals and to reduce the internal dissipation, it has been proposed in a co-pending application for the stabiliser to comprise a resistor and a second transistor, whose main current path is connected in series with the resistor and the main current path of the first transistor, and for the base of the second transistor circuit to be connected to a tap on the voltage divider.
The terminals of such a voltage stabiliser are connected to the subscriber's line via the hook contact. When the receiver is removed from the cradle a charging current is caused to flow from the supply source of an exchange to the capacitor via the subscriber's line and to the voltage divider portion which is connected between the first terminal and the base of the first transistor circuit. In combination with the said portion of the voltage divider, the capacitor forms a low-pass filter which should be capable of attenuating low frequency alternating voltages and not form a load for the line. Hence, the low-pass filter has a very long time constant. In addition, the subscriber's line is capacitive by nature, which opposes line voltage variations.
It is, however, desirable that the voltage stabiliser attains and maintains a stable final value very shortly after switch-on.
It is an object of the invention to provide in a very simple manner a very short switch-on period for the voltage stabiliser without an increase in the sensitivity of the stabiliser to disturbances.